Planar heating units are used as a heat source for heating equipment, cooking appliances, and driers and are attracting attention because they meet such requirements as the reduction of apparatus thickness and uniform heating.
Requirements which planar heating units should meet are as follows.
(1) Superior function of radiating far infrared rays, and high efficiency of energy utilization. PA0 (2) Superior processing dimensional accuracy. PA0 (3) Low heat capacity. PA0 (4) Easy of leading out the terminals. PA0 (5) Capable of uniformly heating objects. PA0 (6) High heat resistance and moisture resistance. PA0 (7) Superior electrical characteristics (insulation resistance and dielectric breakdown strength). PA0 (8) Little variation in the resistance value of heating conductors.
Most of the conventional planar heating units are in the form of a mica or other insulation base plate having a heater wound thereon and are poor in transmission of heat to heating loads, and since their electric heating material is not sealed, there has been a problem in their moisture resistance.
There is another form of planar heating unit wherein a nonsintered sheet, such as alumina, is formed with an electrically conductive pattern using a conductor paste, such as tungsten, with a sheet stuck thereto, and the assembly is sintered. This heating unit is suitable for applications requiring high heat value, but presents such problems as high heat capacity which results in a long heat-up time, and high sintering temperature which makes it difficult to lead out the electrodes because of the melting of contact material.
There are other forms of heating units including one in which an electrically conductive pattern formed between silicone resin, polyimide or other organic films and the heating unit is constructed as by lamination, but these heating units are limited in heat resistance temperature to 250.degree. C. and their service life is also limited.